Hitler's insanity was largely due to belladonna laced pills that the quack "doctor" that he stood by gave him. He suffered from extreme gas (ironic really) that he was taking the pills to "cure" him of.

Europe would be all Germany if it wasnt for him attacking the USSR, which was the worst tactical decision he could have made.

Although tensions were high between the two countries, they signed a peace treaty after world war 1.

The problem wasn't attacking the USSR, the problem was attacking it alone, with less troops and in the middle of the winter. This all happened because he had to help Mussolini who attacked Greece and got slaughtered. Even though in has been tried MANY times no one seems to understand that attacking Russia in the winter is a dead sentence. Also not having reinforcements from Franco didn't help.

The Russians lost 25 million civilians and soldiers fighting the Germans. The US lost under half a million. I think that speaks for itself if we're discussing who can take most credit for defeating the nazis.

The Germans were dug in to Europe. Once they declared war on USSR they lost most of their troops in The Battle of Stalingrad, and they didn't have the resources or manpower to keep the war going, thus the allies got their foothold.

The United States did get France, yes, But the western front was largely a combined effort by the allied forces. The USSR winning that battle was the major turning point in the war.